batman-adv: fix kernel crash due to missing NULL checks

batadv_softif_vlan_get() may return NULL which has to be verified
by the caller.

Fixes: 35df3b29

 ("batman-adv: fix TT VLAN inconsistency on VLAN re-add")
Reported-by: Ryan Thompson <ryan@eero.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>